554 error fix?

This means that you, or someone abusing your e-mail address, is trying to send a message to an unexisting account on Yahoo mail.

This is a remote error status report. There is nothing wrong on your side, except, maybe, you using an incorect address when sending some e-mail.

If you did NOT send the messages generating the error, it means that a spamer, somewhere in the world, is sending spam pretending to be you. There is nothing you can do to make that stop.

Make sure that all your correspondants e-mail addresses are correct.

Electro
September 2011